% !Mode:: "TeX:UTF-8"%確保文檔utf-8編碼
\documentclass[tikz,border=2pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}

\begin{document}

\begin{tikzpicture}[grow =right,
edge from parent/.style={draw,-latex}]
\node[rectangle,draw] (a) {a}
child{ node[rectangle,draw] (b) {b}
child{node[rectangle,draw] (e) {e}}
child{node[rectangle,draw] (f) {f}}
}
child {node[rectangle,draw] (b) {c}
}
;

\end{tikzpicture}
\end{document}